Spectral broadening in conventional and harmonic Doppler measurements with gaseous contrast agents

Gaseous contrast agents can be used to enhance conventional Doppler measurements and to create new harmonic Doppler modalities. The effect of bubble destruction on both conventional and harmonic Doppler spectra was studied using an in vitro, pulsatile flow system. Similar spectrograms were observed for low levels of insonification (below 0.62 MPa) in both conventional and harmonic Doppler measurements of the same flow. As the acoustic output was increased (up to 1.8 MPa) the spectrograms showed marked spectral broadening, especially for harmonic Doppler. The spectral broadening was caused by transit time effects resulting from the destruction of insonated bubbles. This finding was confirmed by performing a similar experiment on stationary polymer-coated microbubbles suspended in a gel phantom. In conclusion, the effect of bubble destruction should be taken into account in both conventional and harmonic Doppler measurements with gaseous contrast agents.